This invention concerns an assembly allowing for connection of optical fibres with optical or optoelectronic components and a process for manufacturing this assembly.
The invention has applications in the area of microelectronics, particularly in all cases where optical fibres are to be connected to laser sources or optical modules (for example dividers, multiplexers or sensors) mounted on optoelectronic substrates.
In the case of telecommunications by optical transmission in particular, the invention can be used when a laser diode (or bar of lateral emission laser diodes) must be connected to an optical fibre (or several optical fibres).
In the area of microelectronics, the increase in functioning frequency of electrical systems requires:
designing of new principles of data transmission and particularly paralleling of electric buses allowing for simultaneous transmission of several signals at once and/or
use of light through optical wave guide (integrated wave guides or optical fibres) in order to increase the information rate.
These optical wave guides provide very good immunity to electromagnetic disturbances.
In addition, optical transmission requires the making of emission, reception and luminous signal processing modules. Techniques have been developed for this, particularly on glass or silicon in order to provide:
coupling of the optical fibres
optical and electrical connections of optoelectronic components
electric connection of electronic interface components. The following documents can be consulted:
[1] xe2x80x9cSoldering technology for optoelectronic packagingxe2x80x9d, 1996 Electronic Component and Technology Conference, p. 26 to 6
[2] xe2x80x9cPassive alignment for optoelectronic components, Advances in electronic packaging, EFP-vol. 19-1, 1997, vol. 1, p. 753 to 758.
[3] xe2x80x9cSilicon motherboards for multichannel optical modulexe2x80x9d, IEEE transactions on components, packaging and manufacturing technology, Part A, vol. 19, no. Mar. 1, 1996, p. 34 to 40.
[4] xe2x80x9cFlip-chip optical fibre attachment to a monolithic optical receiver chipxe2x80x9d, SPIE, vol. 2613, p. 53 to 58.
Each of the assemblies known by documents [1] to [4] is generally in the form of a substrate on which:
optical fibres are connected either facing optical wave guides formed in the substrate or facing laser diodes and/or photodetectors,
optoelectronic components inserted in this substrate or placed on its surface are coupled with optical wave guides and/or optical fibres,
electronic components addressing or collecting information coming from optoelectronic components are positioned.
The optical fibres and optical or optoelectronic components must be perfectly aligned with each other to minimise optical losses.
The degree of precision sought can be less than 0.5 xcexcm.
Two techniques are primarily used for this.
1) A known technique for active alignment of an optical fibre with a laser diode which ensures alignment by electric measurement with a photodiode in real time.
To do this, the laser diode is supplied and a measurement of the luminous power at the exit of the fibre gives an indication of its alignment relative to the laser diode. Optimisation of the alignment is provided by small movements of this laser diode using mechanical or piezoelectric micro-manipulators. An assembly can then be made by bonding.
This active alignment technique has some drawbacks:
long alignment process
the need for mechanical fixing, with glue for example, of the laser diode after alignment and
that this fixing must not produce mechanical stresses which could modify the alignment.
2) A passive alignment technique for optical fibres is also know. Its main objective is cost reduction.
If the fibre must be connected in parallel to a substrate in silicon for example, the most widespread connection method at present is the making of a V-shaped cavity in the silicon substrate which acts as a optical micro-bench, for example according to the principle of etching along the preferential crystalline planes (100).
The fibre is fixed and glued to the bottom of the cavity facing an optoelectronic component. This optoelectronic component, if it is brought onto the substrate, is generally mounted head down using the flip-chip technique on metallic links providing electrical continuity, mechanical resistance and thermal evacuation to the substrate.
The optoelectronic component facing the fibre must be in absolute alignment in the three directions in space. The following can be used for this:
very precise equipment allowing for positioning and welding of the component, while holding it on the substrate, with precision on the order of 1 xcexcm,
soldering joints with positioning shims made in the substrate and/or in the component to be assembled,
soldering joint elements without shims using the auto-positioning effect linked to the wettability forces of the solder in the liquid phase, the auto-positioning taking place in parallel on the substrate by wetting on the metallic studs and perpendicular to the substrate by control of the volume of the soldering elements.
In addition to the use of V-shaped cavities in the substrate, there is a method for attaching by gluing fibres in an intermediate support in silicon, also etched into a V-shape, and then put on the substrate upside down (see document [4]). The alignment and soldering of the intermediate support is done with precision equipment. The auto-alignment effect in the liquid phase is not used. The rigidity of the fibres and the weight of the unit do not allow this.
When a fibre is connected perpendicularly to a substrate, there is a method for inserting the fibre into a pierced block which is first soldered and mounted by the flip-chip method using the auto-alignment effect in the liquid phase. FIG. 5 of document [1] may be consulted for this subject.
There is a problem in the case of passive alignment of an optical fibre facing an optoelectronic component put onto an interconnection substrate.
When the fibre is attached parallel to the substrate in a V-shaped cavity integrated into this same substrate, the optical positioning of the optoelectronic component facing the fibre must be absolute in the three directions in space.
In a plane parallel to the substrate, the metallic studs made, which are soldered, are perfectly aligned with the V-shaped cavity because they are generated by the same lithographic template. The effect of auto-alignment in the liquid phase of the solder microspheres ensures good positioning of the component facing the fibre.
In a direction perpendicular to the substrate however, control of the height of the optical axes requires, for the fibre, control of the width of the V-shaped cavity (variation of burying of the fibre) and of the optoelectronic component, either a mechanical block or a control of the solder volume. These operations depend on the technological manufacturing variations.
The purpose of this invention is to define an assembly and its manufacturing process, allowing for very precise passive alignment of one or several optical fibres with one or several optical or optoelectronic components in a relative manner.
This assembly is made using microspheres of a meltable material (solder) on a substrate, which can be an interconnection substrate, which acts as an optical micro-bench.
The meltable material of the microspheres is for example indium or a meltable tin and lead-based alloy or any alloy with a low melting point.
Precisely, this invention concerns firstly an assembly including a substrate and, on this substrate, at least one optical fibre support, at least one optical fibre in the support and at least one optical or optoelectronic component, the optical axis of the fibre and the optical axis of the component being aligned, this assembly being characterised in that the support and the component are attached to the substrate using microspheres of a meltable material, allowing the optical axis of the fibre and the optical axis of the component to be parallel to each other in the same plane perpendicular to a surface of the substrate, and in that the support includes, for each fibre, a V-shaped housing with two walls inclined towards each other, the opening of the xe2x80x9cVxe2x80x9d being located on the side of the support which is not attached to the substrate, and in that the fibre is positioned in the housing, the volume of each microsphere and the housing being determined so that the optical axis of the fibre and the optical axis of the component are parallel to each other in the same plane parallel to the surface of the substrate.
According to a first particular mode of embodiment of the assembly of the invention, the housing goes through the whole support, this housing thus having two lower edges, the distance separating these two lower edges being determined, given the diameter of the fibre and its supporting points in the housing, so that the optical axis of the fibre and the optical axis of the component are parallel to each other in the same plane parallel to the surface of the substrate.
According to a second particular mode of embodiment, the housing does not go through the support.
This invention also involves a process for manufacturing an assembly including a substrate and, on this substrate, at least one optical fibre support, at least one optical fibre in the support and at least one optical or optoelectronic component, the optical axis of the fibre and the optical axis of the component being aligned, this process being characterised in that the fibre support formed includes, for each fibre, a V-shaped housing with two walls inclined towards each other, the opening of the xe2x80x9cVxe2x80x9d being located on the side of the support which is not attached to the substrate, and also including several first attaching studs, and in that there are also several first attaching studs on the component, in that second attaching studs are formed on the substrate, these second studs being respectively associated with the first studs, in that there are formed on these first studs and/or second studs, elements made of a meltable material apt to be soldered to the first and second studs, these first and second studs being wettable by this material in the molten state while their environment is not, in that the support and the component are attached to the substrate by the corresponding elements, these elements being melted for this purpose and allowing the optical axis of the fibre and the optical axis of the component to be parallel to each other in the same plane perpendicular to a surface of the substrate, and in that the fibre in positioned in the housing, the volume of each element and the housing being determined so that the optical axis of the fibre and the optical axis of the component are parallel to each other in the same plane parallel to the surface of the substrate.
According to a first mode (xe2x80x9cCrossing Vxe2x80x9d) of applying the process of the invention, the housing crosses the whole support, the housing thus having two lower edges, the distance separating these two lower edges being determined, given the diameter of the fibre and its supporting points in the housing, so that the optical axis of the fibre and the optical axis of the component are parallel to each other in the same plane parallel to the surface of the substrate.
According to a second mode of application (xe2x80x9cNon-crossing Vxe2x80x9d), the housing does not go through the support.
The support may include several parallel V-shaped housings with several optical fibres positioned in these housings.
Each fibre may be attached by glue in its corresponding housing.
The glue should preferably be spread between the support and the substrate around the meltable material elements placed on the support to give the assembly greater solidity.
According to a particular mode for applying the invention process, a cap to cover each housing is made and each fibre is pressed against the corresponding housing with this cap and then advantageously attached in the corresponding housing.
This cap may be transparent, allowing for observation of each fibre in its housing and even gluing of this fibre in this housing by hardening of polymerisable glue by radiation (generally ultraviolet radiation) shown on the glue through the cap.
According to a first particular mode for applying the invention process, the cap includes, for each fibre, a protuberance by which this fibre is pressed against the corresponding housing.
According to a second particular mode for applying the invention process, the cap includes a plane side by which each fibre is pressed against the corresponding housing, the fibre going beyond the side of the support which is not attached to the substrate.
According to a particular mode of embodiment of the invention for the xe2x80x9cCrossing Vxe2x80x9d, a support is formed from a plate in which at least two V-shaped walls are made by chemical and/or mechanical etching of this plate in order to obtain a determined distance between the two lower edges of each V. The first studs corresponding to the support are formed by a photolithography technique and the two walls of the housing are separated.
In the particular case where the plate is initially too thick, the formation of the support includes a step of thinning the plate done before or after the etching of the V.
According to another particular mode of embodiment of the invention for the xe2x80x9cNon-crossing Vxe2x80x9d, a support is formed from a plate in which at least two V-shaped walls are made by chemical and/or mechanical etching of this plate and the first studs corresponding to the support are formed by a photolithography technique.
The first studs corresponding to the component are preferably formed by a photolithography technique.
The second studs corresponding to the component are also preferably formed on the substrate by a photolithography technique.
According to a preferred mode of embodiment for the invention, the elements formed on the two studs all have the same thickness.
The elements formed on the second studs are preferably brought to a molten state, each element taking the form of a sphere. These elements in the form of spheres are then brought to the solid state and then to the molten state to assemble the component and the support with the substrate.
The shape of the spheres depends on the shape of the second studs and is thus not necessarily spherical.
In order to have the same degree of precision for the height of the various spheres, the elements are preferably formed simultaneously by photolithography on the second studs.